TUNING CONTROLLERS ON DISTILLATION-COLUMNS WITH THE DISTILLATE-BOTTOMS STRUCTURE

Up until recently, the distillate-bottoms (D-B) control structure was not considered to be a viable scheme for controlling a two-product distillation column. However, Finco et al. demonstrated by simulations and plant tests that it outperforms conventional structures when implemented on distillation columns separating a low relative volatility mixture into high-purity products. The D-B structure has indeterminant steady-state gains, and because of that, it is not possible to derive the transfer function matrix of the D-B control structure by using standard identification techniques. In this work, the D-B matrix is found by appropriately transforming the transfer function matrix of another control scheme. Most of the analysis is based on the assumption that the distillate-vapor (D-V) matrix is available. If the reflux-vapor (R-V) matrix is known instead of the D-V matrix, the necessary modifications in the analysis are also discussed. The methodology is extended to a three-product, sidestream distillation column. The full transfer function matrices of the distillate—sides tr earn—b ottoms (D-S-B) and the D-NS-B structures are derived by using the same ideas as for the D-B structure in the two-product column. © 1990, American Chemical Society. All rights reserved.